OCR GCSE Chemistry A (Gateway)
OCR GCSE Chemistry A C3.2 Energetics Exam Questions
Free OCR GCSE Chemistry A exam questions for C3.2 Energetics, with a mark scheme for every pack. Covers exothermic and endothermic reactions, reaction profiles, activation energy, temperature change experiments, and bond energy calculations.
C3.2 Energetics exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR GCSE Chemistry A C3.2 Energetics
C3.2 covers the energy changes that happen during chemical reactions. On the OCR Gateway Chemistry A course it is part of Topic C3 Chemical reactions, assessed in Paper 1 (Foundation) or Paper 3 (Higher). You classify reactions as exothermic or endothermic, interpret reaction profiles and calculate energy changes using bond energies. It links back to the particle model and bonding in C1 and C2, and forward to catalysts and rates in C5.2 and fuels in C6. Examiners commonly ask for labelled reaction profiles, explanations in terms of bond breaking and bond making, and multi-step bond energy calculations.
What you need to know
- Exothermic reactions Exothermic reactions transfer energy to the surroundings, so the temperature of the surroundings rises. Examples include combustion, neutralisation and many oxidation reactions.
- Endothermic reactions Endothermic reactions take in energy from the surroundings, so the temperature falls. Examples include thermal decomposition and dissolving some salts such as ammonium nitrate.
- Reaction profiles A reaction profile shows the energy of reactants and products and the activation energy, the minimum energy needed for particles to react. In an exothermic profile products are lower than reactants; in an endothermic profile products are higher.
- Bond breaking and making Breaking bonds is endothermic and making bonds is exothermic. If more energy is released making bonds than is taken in breaking them, the reaction is exothermic overall.
- Bond energy calculations Energy change = total energy needed to break bonds in reactants - total energy released making bonds in products. A negative value means the reaction is exothermic.
- Measuring temperature change Mix reactants in an insulated polystyrene cup with a lid, measure the starting and highest or lowest temperature, and repeat. Insulation reduces energy transfer to the surroundings.
Common mistakes in C3.2 Energetics
- Saying bond breaking releases energy. Breaking bonds always requires energy; making bonds releases energy.
- Subtracting the wrong way round in bond energy calculations. Use bonds broken minus bonds made.
- Drawing the activation energy arrow from the products to the peak. It goes from the reactants to the top of the curve.
- Forgetting to count all bonds, for example the two O=O bonds in 2O2 or all four C-H bonds in methane.
- Saying that in an endothermic reaction the reaction mixture gets colder because it loses heat. It gets colder because energy is taken in from the surroundings.
Try three C3.2 Energetics exam-style questions
1. Use the bond energies to calculate the energy change for H2 + Cl2 -> 2HCl. H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, H-Cl = 432 kJ/mol. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 243 = 679 kJ; bonds made = 2 x 432 = 864 kJ; energy change = 679 - 864 = -185 kJ/mol.
2. State whether the reaction in the previous question is exothermic or endothermic and explain why. (2 marks)
Show the mark scheme answer
Exothermic; more energy is released making new bonds than is taken in breaking bonds (energy change is negative).
3. Draw and label a reaction profile for an exothermic reaction. (3 marks)
Show the mark scheme answer
Products shown at lower energy than reactants; curve rising to a peak between them; activation energy labelled from reactants to the peak and overall energy change labelled between reactants and products.
C3.2 Energetics FAQs
What does OCR GCSE Chemistry A C3.2 Energetics cover?
It covers exothermic and endothermic reactions, reaction profiles, activation energy, measuring temperature changes and bond energy calculations.
Which paper is C3.2 examined in?
As part of Topic C3, it is assessed in Paper 1 (Foundation) or Paper 3 (Higher).
How do I avoid mistakes in C3.2 bond energy questions?
Draw out the displayed formula of every molecule, count each bond type, multiply by the number of molecules in the equation, then calculate bonds broken minus bonds made.
